To date, the most successful materials for producing light emitting devices or “LEDs” (including light emitting diodes, laser diodes, photodetectors and the like) capable of operation in the UV, blue and green portions of the electromagnetic spectrum have been the group III-nitride compound semiconductor materials, and in particular gallium nitride-based compound semiconductors. However, gallium nitride presents a particular set of technical problems in manufacturing working devices. The primary problem is the lack of bulk single crystals of gallium nitride which in turn means that gallium nitride or other group III-nitride devices must be formed as epitaxial layers on other materials. Sapphire (i.e., aluminum oxide or Al2O3) has been commonly used as a substrate for group III-nitride devices. Sapphire offers a reasonable crystal lattice match to Group III nitrides, thermal stability, and transparency, all of which are generally useful in producing a light emitting diode. Sapphire offers the disadvantage, however, of being an electrical insulator. This means that the electric current that is passed through an LED to generate the emission cannot be directed through the sapphire substrate. Thus, other types of connections to the LED must be made, such as placing both the cathode and anode of the device on the same side of the LED chip in a so-called “horizontal” configuration. In general, it is preferable for an LED to be fabricated on a conductive substrate so that ohmic contacts can be placed at opposite ends of the device. Such devices, called “vertical” devices, are preferred for a number of reasons, including their easier manufacture as compared to horizontal devices.
In contrast to sapphire, silicon carbide can be conductively doped, and therefore can be effectively used to manufacture a vertical group III-nitride LED. In addition, silicon carbide has a relatively small lattice mismatch with gallium nitride, which means that high-quality group III-nitride material can be grown on it. Silicon carbide also has a high coefficient of thermal conductivity, which is important for heat dissipation in high-current devices such as laser diodes.
Examples of silicon carbide-based group III-nitride LEDs are shown in U.S. Pat. Nos. 5,523,589, 6120,600 and 6,187,606 each of which is assigned to Cree, Inc., the assignee of the present invention, and each of which is incorporated herein by reference. Such devices typically comprise a silicon carbide substrate, a buffer layer or region formed on the substrate, and a plurality of group III-nitride layers forming a p-n junction active region.
In particular, U.S. Pat. No. 6,187,606 represents an important advance over the state of the art as it previously existed. The invention described in the '606 patent provided a plurality of discrete crystal portions, or “dots,” of GaN or InGaN on the substrate in an amount sufficient to minimize or eliminate the heterobarrier between the substrate and the buffer layer. A highly conductive path between the substrate and the active region could thereby be established.
An important parameter for LEDs is the forward voltage (Vf) drop between the anode and the cathode of the device during forward bias operation. In particular, it is desirable for the forward voltage (Vf) of a device to be as low as possible to reduce power consumption and increase the overall efficiency of the device. Despite the advance of the '606 patent, there remains a measurable voltage drop at the interface between a conventional silicon carbide substrate and the conductive buffer layer. It is desirable to reduce this voltage drop in order to reduce the overall Vf of the resulting device.